The present invention concerns a silicate dental ceramic-based blasting medium for the improvement of the adhesive bond between a fired-on ceramic and alloy.
According to the state of the art, individual metal substructures for prosthetic teeth are manufactured according to the investment casting [lost wax] process. In this process, the eventual metal substructure is first modeled in a 1:1 wax scale model, is provided with sprue channels and is embedded in a commercially available fireproof embedding compound in a ring muffle.
After the setup reaction of the embedding compound, the ring muffle is heated in a suitable burn-out oven in order to completely expel the wax and create a hollow mold. A molten dental alloy is introduced into this mold using a centrifugal caster.
After cool-down and removal of the embedding compound from the metal dental substructure, it is finished with rotary tools and checked for conformity to the master model. As a rule, blasting with a blasting medium comes next, followed by an oxidation process and an additional blasting step. Predominantly aluminum oxide (Al.sub.2 O.sub.3) with a particle size of about 110 .mu.m is used as the blasting medium. The purpose of this surface treatment is the improvement of the adhesive bond between the first fired-on ceramic layer (opaque) and the alloy by retention (roughened surface) as well as by formation of adhesive oxides (oxide firing). By application of the dentin core as well as the compound for the cutting edge, each followed by oven firings, the natural tooth is replicated in the end to be as natural looking as possible.
In order to guarantee a good bond between alloy and ceramic, a so-called adhesive firing is also recommended by dental ceramic manufacturers. As in the normal process, a thin opaque layer is first applied to the alloy and then fired at higher temperatures, specifically about 30-50.degree. C. above the normal firing temperature. In this way, the opaque is converted into a low viscosity state, whereby all cavities and uneven places are wetted. After cool-down, a glassy, smooth ceramic coating forms on the metal substructure. Depending on the manufacturer, the adhesive firing can also be executed with a compound offered for this which is characterized as "adhesive bond"--a special bonding agent.
An overfiring of the opaque is unproblematic as far as heat resistance, substructure deformation and fitting are concerned, since, in spite of increased firing temperatures (e.g. 980.degree. C.), the interval to the solidus point of the alloy amounts to more than 200.degree. C. However, it appears differently in the so-called bio-alloys which have been present on the market for several years. This alloy type has a gold portion of over 75% by weight and is correspondingly gold colored. As a result, alloy technology requires the solidus point to be lowered 250 to 350.degree. C. in relation to the classic dental alloys to an absolute value between 900 and 1000.degree. C. The bio-alloys are faced with specially developed, low-melting-point ceramics in order to guarantee the substructure stability and compatibility of the thermal expansion coefficient.
An adhesive firing analogous to the technique of overfiring typical for the classic dental alloys increases the probability of deformation in the dental substructure and for this reason is executed at a correspondingly low temperature for the bio-alloys.
However, in combination with the practice, which was introduced for these new alloy types, of "wearing the prosthetic tooth on a trial basis", an increased bubble formation, compared to the classic metal-ceramic system, is observed in the finish firing. In this process, the prosthetic tooth is temporarily installed in the patient's mouth for a certain period of time (from a few days to a few months) by the dentist for the purpose of reviewing the bio-compatibility and occlusion. After this trial time, the dental work (crowns or bridges) goes back to the dental technology laboratory and is corrected according to dental technology processes and lastly is subjected to a finish or glaze firing.
In the blasting of the metal substructure, residues of the blasting medium remain on or in the surface of the alloy and can be detected by scanning electron microscopic examination and x-ray fluorescence analysis. It is assumed that the additional steps of surface conditioning for the formation of residual pore channels between alloy and first ceramic layer (opaque or adhesive bond) are required with the use of Al.sub.2 O.sub.3 blasting medium, but also with other inert hard materials such as silicon carbide (SiC) or boron carbide (B.sub.4 C) fundamentally suited for this. Moisture is sucked into the channels by capillary forces and the bubbles observed many times in practice after wearing the prosthesis on a trial basis are generated in the subsequent correction firing.
Surprisingly, it was discovered that, after the replacement of the customarily used blasting medium with a silicate dental ceramic-based blasting medium, the bubble formation after wearing of the prosthetic teeth on a trial basis and after subsequent correction firing can be completely suppressed. The disadvantage of the metal-ceramic bio-alloys, compared to the traditional high-melting-point alloy--facing ceramic systems, is thus eliminated.